Oxidation process for the production of carboxylic acids and alkenes

ABSTRACT

An oxidation process for the production of alkenes and carboxylic acids from a feed comprising alkene and/or alkane, carbon monoxide, a molecular oxygen containing gas and optionally water in the presence of an oxidation catalyst in which the level of carbon monoxide is maintained between 1% and 20% by volume of the total feed to the reactor.

This application is the U.S. National Phase of International ApplicationPCT/GB04/002069, filed 13 May 2004, which designated the U.S.PCTIGB04/002069 claims priority to British Application No. 0312965.7filed 5 Jun. 2003. The entire content of these applications areincorporated herein by reference.

The present invention relates to a process for the oxidation of a C₂ toC₄ alkane and/or alkene to produce the corresponding alkene and/orcarboxylic acid and, in particular, to a process for the oxidation ofethane to ethylene and acetic acid. The present invention also relatesto integrated processes in which said alkene and carboxylic acid areused as reactants for the production of alkenyl carboxylates or alkylcarboxylates.

The catalytic gas phase oxidation of ethane to ethylene and acetic acidis known. In 1978, Union Carbide Corporation published a report in theJournal of Catalysis describing a fixed bed process for the oxidation ofethane to ethylene. In addition, several U.S. Pat. Nos. (4,250,346,4,524,236, 4,568,790, 4,899,003 and 4,596,787) describe the lowtemperature oxydehydrogenation of ethane to ethylene. U.S. Pat. No.4,899,003 describes a process for the oxydehydrogenation of ethane inwhich the product stream comprises ethylene, acetic acid, carbon oxidesand unreacted ethane. CO and CO₂ are removed prior to recycling theunreacted ethane. Carbon monoxide may be removed, for example, byoxidation to carbon dioxide and subsequent adsorption.

EP-A-0546677 describes a process for the oxidation of ethane to aceticacid in a fluidised bed reactor. In the process described inEP-A-0546677 most of the reactor effluent is recycled to the reactor inorder to maintain a high concentration of carbon oxides as diluents inthe reactor. (These diluents help to control the temperature.) However,a purge stream is taken from the reactor effluent to prevent a continualbuild-up of carbon oxides in the reactor. In the example of EP-A-0546677the feed to the reactor contains 25% CO and over 40% CO₂. Onedisadvantage of high CO and CO₂ concentrations in the feed is that thereis a reduction in the partial pressures of ethane and ethylene, whichmay reduce the rate of the oxidation reaction.

WO 01/90042 and WO 01/90043. both disclose an integrated process for theproduction of vinyl acetate, the first step of which is the oxidation ofethane to acetic acid and ethylene, with subsequent conversion of theacetic acid and ethylene to vinyl acetate. Carbon monoxide may beproduced as a by-product in the conversion step to vinyl acetate. The COmay be recycled to the oxidation reactor, however, the concentration ofcarbon monoxide in the recycled feed is relatively low. Specifically, WO01/90042 and WO 01/90043 disclose that usually low amounts of carbonmonoxide (<100p pm) are formed in the acetic acid and ethyleneproduction step(s), and that if carbon monoxide is produced in higheramounts (up to 5%), then a CO removal step may be required. WO 01/90042and WO 01/90043 do not disclose any benefits of maintaining an amount ofCO in the feed to the reactor.

EP-A-0 877 727 discloses an integrated process for the production ofacetic acid and/or vinyl acetate in any pre-determined and variableproportions from a gaseous feedstock comprising ethylene and/or ethane.The integrated process comprises a first step wherein ethylene and/orethane is catalytically oxidised in a first reaction zone to produce afirst product stream comprising acetic acid, water and ethylene andoptionally ethane, carbon monoxide and/or carbon dioxide. The aceticacid and ethylene produced in this first reaction zone are thencontacted in a second reaction zone with a molecular oxygen-containinggas in the presence of a catalyst to produce a second product streamcomprising vinyl acetate, water, acetic acid and optionally ethylene.

It has now been found that the oxidation of a C₂ to C₄ alkane to producethe corresponding alkene and carboxylic acid, such as the oxidation ofethane to ethylene and acetic acid, and/or the oxidation of a C₂ to C₄alkene to produce the corresponding carboxylic acid, such as theoxidation of ethylene to acetic acid can be advantageously operated bymaintaining the amount of carbon monoxide in the feed within a definedrange.

Accordingly, in a first aspect, the present invention provides a processfor the oxidation of a C₂ to C₄ alkane to produce the correspondingalkene and carboxylic acid and/or for the oxidation of a C₂ to C₄ alkeneto produce the corresponding carboxylic acid, which process comprisesfeeding to an oxidation reaction zone said alkane and/or alkene, amolecular oxygen-containing gas, carbon monoxide, and optionally water,in the presence of a catalyst active for the oxidation of the alkane tothe corresponding alkene and carboxylic acid and/or active for theoxidation of the alkene to the corresponding carboxylic acid, to producea first product stream comprising alkene and carboxylic acid,characterised in that said carbon monoxide is maintained at between 1%and 20% by volume of the total feed to the oxidation reaction zone.

In a second aspect the present invention relates to an integratedprocess for the production of an alklenyl carboxylate. Accordingly, thepresent invention also provides an integrated process for the productionof alkenyl carboxylate from a C₂ to C₄ alkane and/or a C₂ to C₄ alkene,which process comprises:

-   (a) feeding to an oxidation reaction zone said alkane and/or alkene,    a molecular oxygen-containing gas, carbon monoxide, and optionally    water, in the presence of a catalyst active for the oxidation of the    alkane to the corresponding alkene and carboxylic acid and/or active    for the oxidation of the alkene to the corresponding carboxylic    acid, to produce a first product stream comprising alkene and    carboxylic acid, and wherein the carbon monoxide is maintained at    between 1% and 20% by volume of the total feed to the oxidation    reaction zone, and-   (b) contacting in a second reaction zone at least a portion of said    alkene and at least a portion of said carboxylic acid obtained from    the oxidation reaction zone, and a molecular oxygen-containing gas,    in the presence of at least one catalyst active for the production    of alkenyl carboxylate to produce a second product stream comprising    alkenyl carboxylate.

In a third aspect the present invention relates to an integrated processfor the production of an alkyl carboxylate. Accordingly, the presentinvention also provides an integrated process for the production ofalkyl carboxylate from a C₂ to C₄ alkane and/or a C₂ to C₄ alkene, whichprocess comprises:

-   (a) feeding to an oxidation reaction zone said alkane and/or alkene,    a molecular oxygen-containing gas, carbon monoxide, and optionally    water, in the presence of a catalyst active for the oxidation of the    alkane to the corresponding alkene and carboxylic acid and/or active    for the oxidation of the alkene to the corresponding carboxylic    acid, to produce a first product stream comprising alkene and    carboxylic acid, and wherein the carbon monoxide is maintained at    between 1% and 20% by volume of the total feed to the oxidation    reaction zone, and-   (b) contacting in a second reaction zone at least a portion of said    alkene, at least a portion of said carboxylic acid obtained from the    oxidation reaction zone and, optionally, water, in the presence of    at least one catalyst active for the production of alkyl carboxylate    to produce a second product stream comprising alkyl carboxylate.

It has now been found that the presence of carbon monoxide (CO) in thefeed for the oxidation of C₂ to C₄ alkanes and/or alkenes suppresses theformation of further carbon monoxide and reduces the overall selectivityto carbon oxides (CO_(x)). In addition, the selectivity to the desiredalkene and/or carboxylic acid product or products is increased. Forexample, using the process of the present invention it has been foundthat in the oxidation of ethane to ethylene and acetic acid, theselectivity to acetic acid product may be increased. As a furtheradvantage, the reduction in CO_(x) production (which is a highlyexothermic process) allows improved heat control thereby allowing theoxidation reaction to be run at higher productivity.

Preferably the feed comprises a C₂ to C₄ alkane. The alkane-containingfeed may also comprise the corresponding alkene. Where the correspondingalkene is fed with the alkane to the oxidation reaction zone, theoxidation reaction may be operated with a higher amount of alkene in thefeed. Conventionally, feeding higher amounts of alkene leads to anincrease in CO_(x) formation. However, with the process of the presentinvention, the CO_(x) formation at higher alkene feed amounts(concentrations) is at least partly suppressed by feeding from 1 to 20%by volume of CO. Hence the present invention advantageously allowsincreased alkene to be fed to the oxidation reaction zone, resulting inenhanced productivity. For example, in the oxidation of ethane andethylene to acetic acid and ethylene, it has been found that higheramounts of ethylene may be fed to the reactor to give increasedproductivity.

Preferably, the amount of carbon monoxide in the feed is maintained suchthat the selectivity to carbon monoxide in the oxidation reaction zoneis low, for example, less than 1%. Thus, the amount of carbon monoxidein the first product stream exiting the oxidation reaction zone will notbe significantly higher than the amount of carbon monoxide in the feedto the oxidation reaction zone. Carbon monoxide in the first productstream may be separated from the alkene and carboxylic acid componentsof the first product stream and recycled. Preferably, the carbonmonoxide fed to the oxidation reaction zone comprises at least a portionof said recycled carbon monoxide. More preferably essentially all, suchas 90% or more, more preferably, 95% or more, of the carbon monoxide inthe first product stream may be recycled. Typically, a small amount ofcarbon monoxide will be formed in the oxidation reaction zone and may bebalanced, for example, by losses in the purification stages and/or by apurge stream. Although in theory it is possible to operate with no netcarbon monoxide production (zero selectivity to carbon monoxide) in theoxidation reaction zone, in practice a small purge stream will usuallybe required to prevent build-up of inerts that may be present asimpurities in any of the feeds. This preferred carbon monoxide amount(equilibrium amount) will depend on the specific oxidation reaction, thecatalyst and the reaction conditions, such as temperature. Whenoperating at this equilibrium amount the process of the presentinvention has the further advantage that it may be operated without anyspecific carbon monoxide removal steps (other than any purge), therebyreducing the capital and operating expenditure associated with suchsteps.

Fresh carbon monoxide may also be fed to the oxidation reaction zone.For example, where it is desirable to operate the oxidation reactionzone such that the carbon monoxide in the feed is consumed to formcarbon dioxide at a rate greater than carbon monoxide is produced, freshCO may be fed to the oxidation reaction zone in addition to recycle CO.

Where the oxidation reaction zone is part of an integrated process, suchas that for the production of alkenyl carboxylate, the carbon monoxidefed to the oxidation reaction zone may comprise carbon monoxide recycledfrom other stages of the integrated process, such as, for example,carbon monoxide recycled, after separation, from the second productstream exiting from the second reaction zone for the production ofalkenyl carboxylate.

On start-up, fresh carbon monoxide may be introduced to the feed to thereaction to give the desired amount of carbon monoxide in the totalfeed. Alternatively, the carbon monoxide amount in the feed to theoxidation reaction zone on start-up may initially be lower than theamount it is desired to maintain, but will build-up to the amount it isdesired to maintain in the feed by recycle of the carbon monoxide formedin the process.

Preferably the amount of carbon monoxide in the feed (as fresh and/orrecycle component) is maintained at an amount above 2.5% by volume ofthe total feed, such as above 5% by volume of the total feed, forexample above 5 vol % to 20 vol % or above 5 vol % to 15 vol % of thetotal feed.

Preferably the amount of carbon monoxide in the feed (as fresh and/orrecycle component) is maintained at an amount below 15% by volume of thetotal feed such as in the range above 5 vol % to below 15 vol % of thetotal feed, for example, above 5 vol % to 10 vol % of the total feed.

Preferably the process for the oxidation of a C₂ to C₄ alkene to producethe corresponding carboxylic acid is a process for the production ofacetic acid from ethylene.

Preferably the process for the oxidation of a C₂ to C₄ alkane to producethe corresponding alkene and carboxylic acid is a process for theproduction of ethylene and acetic acid from ethane. Preferably, ethyleneis fed to the reaction zone with ethane.

Solid catalysts active for the oxidation of the C₂ to C₄ alkane and/oralkene may be used in the form of a fixed or fluidised bed. Preferably,the oxidation reaction is performed heterogeneously with solid catalystsand the reactants in the fluid phase

Catalysts active for the oxidation of alkane to alkene and carboxylicacid may comprise any suitable catalysts known in the art, for example,for the oxidation of ethane to ethylene and acetic acid as described inU.S. Pat. No. 4,596,787, EP-A-0407091, DE 19620542, WO 99/20592, DE19630832, WO 98/47850, WO 99/51339, EP-A-0 1043064, WO 9913980, U.S.Pat. Nos. 5,300,682 and 5,300,684, the contents of which are herebyincorporated by reference.

U.S. Pat. No. 4596787 relates to a process for the low temperatureoxydehydrogenation of ethane to ethylene using a catalyst having theempirical formula Mo_(a)V_(b)Nb_(c)Sb_(d)X_(e) as therein defined, theelements being present in combination with oxygen.

EP-A-0407091 relates to process and catalyst for the production ofethylene and/or acetic acid by oxidation of ethane and/or ethylene inthe presence of an oxidation catalyst comprising molybdenum, rhenium andtungsten.

DE 19620542 relates to molybdenum, palladium, rhenium based oxidationcatalysts for the production of acetic acid from ethane and/or ethylene.

WO 99/20592 relates to a method of selectively producing acetic acidfrom ethane, ethylene or mixtures thereof and oxygen at high temperaturein the presence of a catalyst having the formula Mo_(a)Pd_(b)X_(c)Y_(d)wherein X represents one or several of Cr, Mn, Nb, Ta, Ti, V, Te and W;Y represents one or several of B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co,Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Nb, Zr, Hf,Ni, P, Pb, Sb, Si, Sn, Tl and U and a=1, b=0.0001 to 0.01, c=0.4 to 1and d=0.005 to 1.

German patent application DE 196 30 832 A1 relates to a similar catalystcomposition in which a=1, b>0, c>0 and d=0 to 2. Preferably, a=1,b=0.0001 to 0.5, c=0.1 to 1.0 and d=0 to 1.0.

WO 98/47850 relates to a process for producing acetic acid from ethane,ethylene or mixtures thereof and a catalyst having the formulaW_(a)X_(b)Y_(c)Z_(d) in which X represents one or several of Pd, Pt, Agand Au, Y represents one or several of V, Nb, Cr, Mn, Fe, Sn, Sb, Cu,Zn, U, Ni, and Bi and Z represents one or several of Li, Na, K, Rb, Cs,Be, Mg, Ca, Sr, Ba, Sc, Y, La, Ti, Zr, Hf, Ru, Os, Co, Rh, Ir, B, Al,Ga, In, Tl, Si, Ge, Pb, P, As and Te, a=1, b>0, c>0 and d is 0 to 2.

WO 99/51339 relates to a catalyst composition for the selectiveoxidation of ethane and/or ethylene to acetic acid which compositioncomprises in combination with oxygen the elementsMo_(a)W_(b)Ag_(c)Ir_(d)X_(e)Y_(f) wherein X is the elements Nb and V; Yis one or more elements selected from the group consisting of Cr, Mn,Ta, Ti, B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Cu, Au, Fe, Ru, Os,K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, TI, U, Re andPd; a,,b, c, d, e and f represent the gram atom ratios of the elementssuch that 0<a≦1, 0≦b<1 and a+b=1; 0<(c+d)≦0.1; 0<e≦2;and 0≦f≦2.

EP-A-1043064 relates to a catalyst composition for the oxidation ofethane to ethylene and/or acetic acid and/or for the oxidation ofethylene to acetic acid which composition comprises in combination withoxygen the elements molybdenum, vanadium, niobium and gold in theabsence of palladium according to the empirical formula:Mo_(a)W_(b)Au_(c)V_(d)Nb_(e)Y_(f) wherein Y is one or more elementsselected from the group consisting of: Cr, Mn, Ta, Ti, B, Al, Ga, In,Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Fe, Ru, Os, K, Rb, Cs, Mg, Ca,Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl, U, Re, Te and La; a, b, c, d,e and f represent the gram atom ratios of the elements such that: 0<a≦1;0≦b<1 and a+b=1; 10-5<c≦0.02; 0<d≦2; 0<e≦1; and 0

WO 99/13980 relates to a catalyst for the selective oxidation of ethaneto acetic acid of formula: Mo_(a)V_(b)Nb_(c)X_(d) wherein X is at leastone promoter element selected from the group consisting of P, B, Hf, Teand As; a is a number ranging from about 1 to about 5; b is 1; c is anumber ranging from about 0.01 to about 0.5; and d is a number rangingfrom greater than 0 to about 0.1.

US 5300682 relates to the use of oxidation catalyst with empiricalformula of VP_(a)M_(b)O_(x) where M is one or more of Co, Cu, Re, Fe,Ni, Nb, Cr, W, U, Ta, Ti, Zr, Hf, Mn, Pt Pd, Sn, Sb, Bi, Ce, As, Ag andAu, a is 0.5 to 3, b is 0 1 and x satisfies the valence requirements.

U.S. Pat. No. 5,300,684 relates to a fluid bed oxidation reaction usingfor example Mo_(0.37)Re_(0.25)V_(0.26)Nb_(0.07)Sb_(0.03)Ca_(0.02)O_(x).

Other suitable oxidation catalysts for use in the present invention aredescribed in WO 99/13980 which relates to the use of catalysts withelements in combination with oxygen in the relative gram atom ratios ofMo_(a)V_(b)Nb_(c)X_(d) where X=P, B, Hf, Te or As; U.S. Pat. No.6,030,920 which relates to the use of catalysts with elements incombination with oxygen in the relative gram atom ratios ofMo_(a)V_(b)Nb_(c)Pd_(d); WO 00/00284 which relates to the use ofcatalysts with elements in combination with oxygen in the relative gramatom ratios of Mo_(a)V_(b)Nb_(c)Pd_(d) and/or Mo_(a)V_(b)La_(c)Pd_(d);U.S. Pat. No. 6,087,297 which relates to the use of catalysts withelements in combination with oxygen in the relative gram atom ratios ofMo_(a)V_(b)Pd_(c)La_(d); WO 00/09260 which relates to the use ofcatalysts with elements in combination with oxygen in the relative gramatom ratios of Mo_(a)V_(b)La_(c)Pd_(d)Nb_(e)X_(f) where X=Cu or Cr and eand f can be zero; WO 00/29106 and WO 00/29105 which relate to the useof catalysts with elements in combination with oxygen in the relativegram atom ratios of Mo_(a)V_(b)Ga_(c)Pd_(d)Nb_(e)X_(f) wherein X=La, Te,Ge, Zn, Si, In or W and WO 00/38833 which relates to the use ofcatalysts with elements in combination with oxygen in the relative gramatom ratios of Mo_(a)V_(b)La_(c)Pd_(d)Nb_(e)X_(f) wherein X=Al, Ga, Ge,or Si, the contents of which are hereby incorporated by reference.

Solid catalysts active for the oxidation of the C₂ to C₄ alkane and/oralkene may be supported or unsupported. Examples of suitable supportsinclude silica, diatomaceous earth, montmorillonite, alumina, silicaalumina, zirconia, titania, silicon carbide, activated carbon andmixtures thereof.

Each of the alkane, molecular oxygen-containing gas, alkene and watermay be introduced into the oxidation reaction zone as fresh feed and/orrecycle component.

The molecular oxygen-containing gas used in the oxidation reaction zonemay be air or a gas richer or poorer in molecular oxygen than air. Asuitable gas may be, for example, oxygen diluted with a suitablediluent, for example nitrogen or carbon dioxide. Preferably, themolecular oxygen-containing gas is oxygen. Preferably, at least some ofthe molecular oxygen-containing gas is fed to the oxidation reactionzone independently from the alkane and optional alkene feeds, and anyrecycle streams.

The alkane and/or alkene fed into the oxidation reaction zone of theprocess of the present invention may be substantially pure or may beadmixed, for example, with one or more of nitrogen, methane, carbondioxide, carbon monoxide, hydrogen, and low amounts of C₃/C₄alkenes/alkanes.

Preferably, the total amount of inert impurities, such as methane,nitrogen, carbon dioxide and argon present in the alkene and/or alkanefeed is in the range 0 to 3 vol % and more preferably, in the range 0 to2.5 vol % such as 0 to 2.14 vol %.

Preferably, the total amount of reactive impurities, such as propane andother hydrocarbons, present in the alkene and/or alkane feed is in therange 0 to 10 vol % and more preferably, in the range 0 to 5 vol %.

Fresh carbon monoxide, if used, may be essentially pure or may containimpurities such as carbon dioxide, hydrogen, nitrogen, noble gases andwater.

Suitably, the concentration of alkane (as fresh feed and recyclecomponent) is from 0 to 90 mol % inclusive of the total feed, includingrecycles, to the oxidation reaction zone, preferably from 10 to 80 mol%, more preferably from 40 to 80 mol %.

Suitably, the concentration of alkene (as fresh feed and recyclecomponent) is from 0 to 50 mol % inclusive of the total feed, includingrecycles, to the oxidation reaction zone, preferably from 1 to 30 mol %,more preferably from 2 to 20 mol %.

Suitably, the concentration of optional water (as fresh feed and recyclecomponent) is from 0 to 50 mol % inclusive of the total feed, includingrecycles, to the oxidation reaction zone, preferably from 0 to 25 mol %,more preferably from 2 to 15 mol %.

A preferred feed to the oxidation reaction zone for the oxidation ofethane and ethylene to acetic acid and ethylene comprises (in mol %)40-80% ethane, 2-20% ethylene, 2-15% water, 5-15% carbon monoxide and5-20% oxygen, with a balance of inert gases, such as argon, carbondioxide and/or nitrogen. The oxygen is preferably added directly in tothe fluidized bed.

When solid catalysts are used in the oxidation reaction zone alkaneand/or alkene, molecular-oxygen containing gas and any recycle gases arepreferably passed through the oxidation reaction zone with a residencetime corresponding to a combined gas hourly space velocity (GHSV) of500-10,000 hr⁻¹; the GHSV being defined as volume (calculated at STP) ofgas passing through the reactor divided by the bulk volume of settledcatalyst.

The oxidation reaction of the present invention may suitably be carriedout at a temperature in the range from 100 to 400° C., typically in therange 200 to 380° C., preferably 250 to 350° C.

The oxidation reaction of the present invention may suitably be carriedout at atmospheric or superatmospheric pressure, for example in therange from 80 to 400 psig.

Typically, alkane conversions in the range 1 to 99% may be achieved inthe oxidation reaction of the present invention.

Typically, oxygen conversions in the range 30 to 100% may be achieved inthe oxidation reaction of the present invention.

In the oxidation reaction of the present invention, the catalystsuitably has a productivity in the range 10 to 10000 grams of carboxylicacid, such as acetic acid, per hour per kilogram of catalyst.

The first product stream from the oxidation process may be fed directlyto a downstream process, but preferably is fed to a downstream processindirectly after one or more separation stages, such as removal ofcarbon monoxide by separation or reaction. Hence, in the second aspectof the present invention, at least a portion of the alkene and at leasta portion of the carboxylic acid obtained from the oxidation reactionzone are contacted with molecular oxygen-containing gas to producealkenyl carboxylate, such as vinyl acetate. In the third aspect of thepresent invention, at least a portion of the alkene and at least aportion of the carboxylic acid obtained from the oxidation reaction zoneare contacted with a suitable catalyst to produce alkyl carboxylate,such as ethyl acetate. Preferably, alkane is oxidized in the presence ofa suitable oxidation catalyst in the first oxidation reaction zone toproduce an approximately 1:1 ratio of alkene and carboxylic acid for usein the subsequent alkenyl carboxylate or alkyl carboxylate reaction.However alkene or carboxylic acid may be added to (or removed from) thefirst product stream as required to give the desired feed to the secondreaction zone. Hence, optional additional alkene and/or optionaladditional carboxylic acid may be added, or carboxylic acid and/oralkene may be recovered from the first product stream prior to thesecond reaction zone.

The additional alkene may be fresh alkene and/or recycled alkene fromthe second reaction zone.

Additional alkene introduced into the second reaction zone for theproduction of alkenyl carboxylate or alkyl carboxylate may besubstantially pure or may be admixed, for example, with one or more ofnitrogen, argon, methane, carbon dioxide, carbon monoxide, hydrogen, andlow amounts of C_(3/)C₄ alkenes/alkanes.

Advantageously, in the second aspect of the present invention, highconcentrations of alkene are fed to the second reaction zone and lowconcentrations of alkene are fed to the oxidation reaction zone. Lowconcentrations (less than 20 mol % of total feed) of alkene fed to theoxidation reaction zone allow the required equimolar or approximateequimolar mixture of alkene and carboxylic acid to be produced. Highconcentrations of alkene (greater than 50 mol % of the total feed) fedto the second reaction zone maximize the selectivity to alkenylcarboxylate product such as vinyl acetate.

Desirably, the concentration of alkene, such as ethylene, fed to thesecond reaction zone in the second aspect of the invention is at least50 mol % of the total feed to the second reaction zone, preferably, atleast 55 mol %, more preferably at least 60 mol %. Suitably, theconcentration of alkene is up to 85 mol % of the total feed to thesecond reaction zone, preferably, in the range at least 50 mol % to 80mol %, such as at least 55 mol % to 80 mol %.

Advantageously, in the third aspect of the present invention optimalconcentrations of alkene are fed to the second reaction zone and lowconcentrations of alkene are fed to the oxidation reaction zone. Lowconcentrations (less than 20 mol % of total feed) of alkene fed to theoxidation reaction zone allow the required equimolar or approximateequimolar mixture of alkene and carboxylic acid to be produced. Optimalconcentrations of alkene may be fed to the second reaction zone tomaximize the selectivity to alkyl carboxylate product, such as ethylacetate.

Desirably, the concentration of alkene, such as ethylene, fed to thesecond reaction zone is at least 50 mol % of the total feed to thesecond reaction zone, preferably, at least 55 mol %, more preferably atleast 60 mol %.

Catalysts known in the art for the production of alkenyl carboxylatesmay be used in the second aspect of the process of the presentinvention. Thus, catalyst active for the production of vinyl acetatewhich may be used in a second reaction zone of the present invention maycomprise, for example, catalysts as described in GB 1 559 540; U.S. Pat.No. 5,185,308 and EP-A-0672453 the contents of which are herebyincorporated by reference.

GB 1 559 540 describes a catalyst active for the preparation of vinylacetate by the reaction of ethylene, acetic acid and oxygen, thecatalyst consisting essentially of: (1) a catalyst support having aparticle diameter of from 3 to 7 mm and a pore volume of from 0.2 to 1.5ml/g, a 10% by weight water suspension of the catalyst support having apH from 3.0 to 9.0, (2) a palladium-gold alloy distributed in a surfacelayer of the catalyst support, the surface layer extending less than 0.5mm from the surface of the support, the palladium in the alloy beingpresent in an amount of from 1.5 to 5.0 grams per litre of catalyst, andthe gold being present in an amount of from 0.5 to 2.25 grams per litreof catalyst, and (3) from 5 to 60 grams per litre of catalyst of alkalimetal acetate. U.S. Pat. No. 5,185,308 describes a shell impregnatedcatalyst active for the production of vinyl acetate from ethylene,acetic acid and an oxygen containing gas, the catalyst consistingessentially of: (1) a catalyst support having a particle diameter fromabout 3 to about 7 mm and a pore volume of 0.2 to 1.5 ml per gram, (2)palladium and gold distributed in the outermost 1.0 mm thick layer ofthe catalyst support particles, and (3) from about 3.5 to about 9.5% byweight of potassium acetate wherein the gold to palladium weight ratioin said catalyst is in the range 0.6 to 1.25.

EP-A-0672453 describes palladium containing catalysts and theirpreparation for fluid bed vinyl acetate processes.

Catalysts known in the art for the production of alkyl carboxylates maybe used in the third aspect of the process of the present invention.Catalysts active for the production of alkyl carboxylates which may beused in the second reaction zone may comprise, for example, catalysts asdescribed in EP-A-0926126; the contents of which are hereby incorporatedby reference.

EP-A-0926126 describes a process for the production of esters byreacting, in a plurality of reactors set up in series, ethylene,propylene or mixtures thereof with a saturated aliphatic C₁-C₄mono-carboxylic acid in the presence of a heteropolyacid catalyst.

Typically, the production of alkenyl carboxylate, such as vinyl acetate,or alkyl carboxylate, such as ethyl acetate, in the second reaction zoneis carried out heterogeneously with the reactants being present in thegas phase.

The molecular oxygen-containing gas used in the second reaction zone forthe production of alkenyl carboxylate may comprise unreacted molecularoxygen-containing gas from the oxidation reaction zone and/or additionalmolecular oxygen-containing gas.

The additional molecular oxygen-containing gas, if used, may be air or agas richer or poorer in molecular oxygen than air. A suitable additionalmolecular oxygen-containing gas may be, for example, oxygen diluted witha suitable diluent, for example nitrogen, argon or carbon dioxide.Preferably, the additional molecular oxygen-containing gas is oxygen.Preferably, at least some of the molecular oxygen-containing gas is fedindependently to the second reaction zone from the alkene and carboxylicacid reactants.

In the third aspect of the present invention water may optionally beadded in the second reaction zone for the production of alkylcarboxylate. When present, the water is suitably present in the form ofsteam and in an amount in the range 1-10 mol % of the total feed to thesecond reaction zone.

The additional carboxylic acid fed to the second reaction zone for theproduction of alkenyl carboxylate or alkyl carboxylate may comprisefresh acid and/or recycle acid. Preferably, at least a portion of thecarboxylic acid introduced in to the second reaction zone comprisescarboxylic acid produced from the oxidation reaction zone.

The fresh and recycle carboxylic acid may be introduced into the secondreaction zone either as separate feed streams or as a single feed streamcomprising both fresh and recycle acid.

The recycle carboxylic acid fed to the second reaction zone for theproduction of alkenyl carboxylate or alkyl carboxylate may comprise atleast a portion of the acid obtained from downstream processes such asfrom the separation of unreacted acid from the second product stream.

At least part of the carboxylic acid fed to the second reaction zone maybe liquid. When solid catalysts are used in the second reaction zone forthe production of alkenyl carboxylate, the alkene, the carboxylic acid,any additional alkene or carboxylic acid reactants, any recycle streamsand molecular oxygen-containing gas are preferably passed through thesecond reaction zone at a combined gas hourly space velocity (GHSV) of500-10,000 hr⁻¹.

The second reaction zone for the production of alkenyl carboxylate maysuitably be operated at a temperature in the range from 140 to 200° C.

The second reaction zone for the production of alkenyl carboxylate maysuitably be operated at a pressure in the range 50 to 300 psig.

The second reaction zone for the production of alkenyl carboxylate maysuitably be operated as either a fixed or a fluidised bed process.

Carboxylic acid conversions in the range 5 to 80% may be achieved in thesecond reaction zone for the production of alkenyl carboxylate.

Oxygen conversions in the range 20 to 100% may be achieved in the secondreaction zone for the production of alkenyl carboxylate.

Alkene conversions in the range 3 to 100% may be achieved in the secondreaction zone for the production of alkenyl carboxylate.

Suitably, the selectivity based on the alkene to the alkenyl carboxylateproduct, such as vinyl acetate of at least 85%, such as at least 90% maybe achieved in the second reaction zone

In the second reaction zone for the production of alkenyl carboxylate,the catalyst suitably has a productivity in the range 10 to 10000 gramsof alkenyl carboxylate per hour per kg of catalyst.

When ethane is used in the process of the second aspect of the presentinvention, the product stream from the second reaction zone for theproduction of alkenyl carboxylate may comprise vinyl acetate, water andacetic acid and optionally also unreacted ethylene, ethane, oxygen,acetaldehyde, nitrogen, argon, carbon monoxide and carbon dioxide. Sucha product stream may be separated by azeotropic distillation into anoverhead fraction comprising vinyl acetate and water and a base fractioncomprising acetic acid and water. The base fraction is removed from thedistillation column as liquid from the bottom of the column.Additionally, a vapour from one or more stages above the bottom of thecolumn may also be removed. Prior to such a distillation step, ethylene,ethane, acetaldehyde, carbon monoxide and carbon dioxide, if any, may beremoved from the second product stream, suitably as an overhead gaseousfraction from a scrubbing column, in which a liquid fraction comprisingvinyl acetate, water and acetic acid is removed from the base. Thecarbon monoxide, if any, may be recycled to the oxidation reaction zone.The ethylene and/or ethane may be recycled to the oxidation reactionzone and/or the second reaction zone.

Vinyl acetate is recovered from the overhead fraction, suitably forexample by decantation. The recovered vinyl acetate may, if desired, befurther purified in known manner.

The base fraction comprising acetic acid and water may be recycled, withor preferably without further purification, to the second reaction zone.Alternatively, acetic acid is recovered from the base fraction and maybe further purified if desired, in known manner, for example, bydistillation.

Where ethane is used in the process of the second aspect of the presentinvention, preferably the acetic acid and ethylene are separated fromthe effluent, including carbon monoxide, from the oxidation reactionzone (ethane oxidation reactor) prior to reaction of the acetic acid andethylene in the second reaction zone (VAM reactor). The remainingeffluent may be treated as desired, for example to remove at least someof the CO₂ produced, and recycled to the oxidation reaction zone tomaintain the carbon monoxide amount in the feed to said reaction zone.The process of the present invention obviates the need for or mitigatesthe scale required for any carbon monoxide removal process steps (forexample, as required in the process disclosed in WO 01/90042).

In another embodiment of the second aspect of the present invention atleast some of the carbon monoxide in the effluent from the oxidationreaction zone may be fed to the second reaction zone to maintain asuitable carbon monoxide amount in the feed to the second reaction zone.Carbon monoxide, including any further carbon monoxide that may beproduced in the second reaction zone, may be subsequently separated fromsecond product stream and recycled to the oxidation reaction zone tomaintain the required carbon monoxide amount in the feed to theoxidation reaction zone.

BRIEF DESCRIPTION OF THE DRAWINGS

The process of the present invention will now be illustrated withrespect to the figures and examples.

FIG. 1 represents, in schematic form, a process for the oxidation ofethane and ethylene to acetic acid and ethylene.

FIG. 2, represents, in schematic form, an integrated process for theproduction of vinyl acetate from ethane and ethylene according to thesecond aspect of the present invention.

With reference to FIG. 1, a feedstream (1), comprising fresh ethane andoxygen, and optional water and ethylene, and a recycle stream (2)comprising unreacted ethane and carbon monoxide are fed to a fluid bedethane oxidation reactor having a bed of suitable oxidation catalyst (3)for the production of acetic acid and ethylene. The feed has 1 to 20% byvolume of carbon monoxide. The oxidation reaction produces a productstream (4) comprising acetic acid, ethane, ethylene, carbon monoxide,carbon dioxide, water and any inert gas that is present in the feedand/or recycle streams. The water and acetic acid are separated in asuitable first separation means (5), for example in a scrubber, toprovide a gaseous stream (6) comprising predominantly ethane, ethylene,carbon monoxide and carbon dioxide. Optionally, water may be removedfrom the acetic acid using a suitable separation means, such asdistillation. At least some of the carbon dioxide from gaseous stream(6) may be removed in a CO₂ removal system (7), for example, usingpotassium carbonate. At least some of any inert gas present and some ofthe ethylene from stream (6) may be recovered by separation in asuitable second separation means (8), to leave a stream (2) comprisingunreacted ethane, carbon monoxide and any remaining inerts, CO₂ orethylene, which is recycled to the ethane oxidation reactor (3), tomaintain the required carbon monoxide amount. A purge may be taken fromthis recycle stream, or elsewhere, to prevent build-up of inerts.

FIG. 2 is generally similar to FIG. 1 and thus the same referencenumerals are used, where appropriate. In FIG. 2, the recovered aceticacid and water stream from the first separation means (5) and therecovered ethylene stream from the second separation means (8), togetherwith oxygen, and optional additional acetic acid and/or ethylene asnecessary are then fed to a vinyl acetate reactor (9), wherein they arecontacted with a suitable catalyst to give a second product streamcomprising vinyl acetate. Optionally, prior to the feeding the aceticacid and water stream from the first separation means to the vinylacetate reactor (9), at least a portion of said acetic acid and waterstream may be fed to a suitable separation means to remove at least aportion of the water therein, for example, in a distillation column. Anycarbon monoxide in the second product stream may be separated andcombined with recycle stream (2) for recycle to the fluid bed ethaneoxidation reactor (3). Any unreacted ethylene and acetic acid, in thesecond product stream may be separated and recycled to the vinyl acetatereactor (9).

EXAMPLES

Carbon monoxide at various amounts was added to a feed to a reactor forthe oxidation of ethane to acetic acid. The feed comprised 60 vol %ethane, 5 vol % ethylene, 5 vol % water, 6 vol % oxygen, the requisiteamount of carbon monoxide, with a balance of nitrogen. The feed waspassed over an ethane oxidation catalyst at temperatures ofapproximately 302° C., 293° C. and 283° C. respectively. The results aregiven in Table 1 below.

TABLE 1 CO in Conversion Selectivity feed (%) (C-mol %) (mole %) OxygenEthylene CO CO₂ COx AcOH 302° C., 16 barg, 3200/h, 60% ethane, 5%ethylene, 5% water, 6.6% oxygen, balance nitrogen. 0 98.0 55.9 9.9 3.813.7 30.4 2.5 96.8 52.4 8.7 4.4 13.1 34.5 5 97.4 51.2 5.0 6.1 11.1 37.610 96.9 52.8 0.7 7.7 8.4 38.8 293° C., 16 barg, 3200/h, 60% ethane, 5%ethylene. 5% water. 6.6% oxygen, balance nitrogen. 0 82.6 55.3 9.8 3.813.6 31.2 2.5 79.5 51.6 8.3 4.2 12.5 35.8 5 82.8 50.3 4.9 6.1 11.0 38.710 81.5 54.2 −1.3 8.3 7.1 38.8 283° C., 16 barg, 3200/h, 60% ethane, 5%ethylene, 5% water, 6.6% oxygen, balance nitrogen. 0 64.5 50.7 10.4 4.114.5 34.8 2.5 60.1 53.1 8.7 4.1 12.8 34.1 5 63.8 47.7 5.0 6.2 11.2 41.110 61.4 51.8 −2.9 8.1 5.2 43.0

These results show that on increasing the amount of CO in the feed tothe reactor the amount of CO formed is suppressed. The suppression of COis not compensated by an equivalent increase in the CO₂ formation,resulting in a net decrease in total carbon oxides (CO_(x)) formationand an increase in acetic acid selectivity.

These results also demonstrate that the point at which CO formation istotally inhibited (no net CO production) is dependent on the temperatureof the oxidation reaction zone. Thus, at a higher temperature a higherconcentration of carbon monoxide in the feed to the oxidation reactionzone is required to completely inhibit CO formation.

1. A process for the oxidation of a C₂to C₄ alkane to produce thecorresponding alkene and carboxylic acid and/or for the oxidation of aC₂ to C₄ alkene to produce the corresponding carboxylic acid, whichprocess comprises feeding to an oxidation reaction zone said alkaneand/or alkene, a molecular oxygen-containing gas, carbon monoxide, andoptionally water, in the presence of a catalyst active for the oxidationof the alkane to the corresponding alkene and carboxylic acid and/oractive for the oxidation of the alkene to the corresponding carboxylicacid, to produce a first product stream comprising alkene and carboxylicacid, characterised in that said carbon monoxide is maintained atbetween 1% and 20% by volume of the total feed to the oxidation reactionzone.
 2. A process according to claim 1 which further comprisescontacting in a second reaction zone at least a portion of said alkeneand at least a portion of said carboxylic acid obtained from theoxidation reaction zone, and a molecular oxygen-containing gas, in thepresence of at least one catalyst active for the production of alkenylcarboxylate to produce a second product stream comprising alkenylcarboxylate.
 3. A process according to claim 1 which further comprisescontacting in a second reaction zone at least a portion of said alkene,at least a portion of said carboxylic acid obtained from the oxidationreaction zone and, optionally, water, in the presence of at least onecatalyst active for the production of alkyl carboxylate to produce asecond product stream comprising alkyl carboxylate.
 4. A processaccording to claim 1 wherein the carbon monoxide is fed to the oxidationreaction zone as a fresh gas and/or as a recycle gas.
 5. A processaccording to claim 1 wherein the first product stream comprises carbonmonoxide.
 6. A process according to claim 5 wherein at least 90% of thecarbon monoxide present in the first product stream is recycled to theoxidation reaction zone.
 7. A process according to claim 2 wherein thesecond product stream comprises carbon monoxide.
 8. A process accordingto claim 7 wherein carbon monoxide is separated from the second productstream and recycled to the oxidation reaction zone.
 9. A processaccording to claim 1 wherein the amount of carbon monoxide in the feed(as fresh and/or recycle gas) is maintained above 2.5% by volume of thetotal feed.
 10. A process according to claim 9 wherein the amount ofcarbon monoxide is maintained above 5% by volume of the total feed. 11.A process according to claim 9 wherein the amount of carbon monoxide ismaintained in the range above 5% by volume to 20% by volume of the totalfeed.
 12. A process according to claim 9 wherein the amount of carbonmonoxide is maintained in the range above 5% by volume to 15% by volumeof the total feed.
 13. A process according to claim 1 wherein the amountof carbon monoxide in the feed (as fresh and/or recycle gas) ismaintained below 15% by volume of the total feed.
 14. A processaccording to claim 13 wherein the amount of carbon monoxide ismaintained in the range above 5% by volume to below 15% by volume of thetotal feed.
 15. A process according to claim 13 wherein the amount ofcarbon monoxide is maintained in the range above 5% by volume to 10% byvolume of the total feed.
 16. A process according to claim 1 wherein theC₂-C₄ alkane is ethane, the C₂ to C₄alkene is ethylene and thecarboxylic acid is acetic acid.
 17. A process according to claim 1wherein ethane and ethylene are fed to the oxidation reaction zone. 18.A process according to claim 1 wherein each of the alkane and alkene isfed to the oxidation reaction zone as fresh feed and/or as a recyclecomponent.
 19. A process according to claim 1 wherein the concentrationof alkane (as fresh feed and recycle component) is from 0 to 90 mol % ofthe total feed to the oxidation reaction zone.
 20. A process accordingto claim 1 wherein the concentration of alkene (as fresh feed andrecycle component) is from 0 to 50 mol % of the total feed to theoxidation reaction zone.
 21. A process according to claim 1 in whichwater is fed to the oxidation zone as fresh feed and/or recyclecomponent in a concentration in the range greater than 0 to 50 mol % ofthe total feed.
 22. A process according to claim 1 wherein the mel ratioof alkene to carboxylic acid in the first product stream isapproximately 1:1.
 23. A process according to claim 2 wherein thealkenyl carboxylate is vinyl acetate.
 24. A process according to claim 2wherein additional alkene and/or additional carboxylic acid is fed tothe second reaction zone.
 25. A process according to wherein theconcentration of alkene fed to the oxidation reaction zone is less than20 mol % of the total feed and/or the concentration of alkene fed to thesecond reaction zone is greater than 50 mol % of the total feed.
 26. Aprocess according to claim 2 wherein the concentration of alkene fed tothe second reaction zone is at least 60 mol % of the total feed.
 27. Aprocess according to claim 25 or claim 26 wherein the alkene isethylene.
 28. A process according to claim 2 wherein the second reactionzone is a fixed bed or a fluidised bed reactor.
 29. A process accordingto claim 3 wherein the alkyl carboxylate is ethyl acetate.
 30. A processaccording to claim 3 or claim 29 wherein water is fed to the secondreaction zone in an amount in the range 1 to 10 mol % of the total feed.31. A process according to claim 1 wherein the oxidation reaction iscarried out at a temperature in the range 100 to 400° C.
 32. A processaccording to claim 1 wherein the total amount of inert impuritiespresent in the alkene and/or alkane feed to the oxidation reactor is inthe range 0 to 3 vol %.
 33. A process according to claim 1 wherein thetotal amount of reactive impurities present in the alkene and/or alkanefeed to the oxidation reactor is in the range 0 to 10 vol %.